According to all that I've read and practiced, all plugs should be indexed to the particular combustion chamber. Not just for piston to electrode strap clearance but to provide best ignition and create a positive flame front for maximum expansion of the fuel/air compound when it ignites . . And each Combustion Chamber is a little different, so to speak.
It's basically the physics of the internal combustion engine and what works best. Higher compressions create problems with ignition, and when we use higher octane (less volatile) fuels, getting the fire started in the right place is what it's all about . .
You must start your plug search with the head on a bench. You can't discern much (except by hit and miss) if you're trying to figure out where the electrode is in the combustion chamber while it's on the engine . . Once you know the reach, and the direction of firing, then you put the head on and start looking at clearances in actual conditions and head gasket dimensions . .
According to most builders and tuners, a plug needs one thread into the combustion chamber. Any more (or less) than that and you're beating a dead horse. Then you must find a plug with the proper electrode reach. Far enough into the chamber to ignite the fresh charge from squish to open . . but not far enough to kill a piston or the plug . .
To some of you, this isn't anything new, to others, helpful I hope . .
New plug technology being used by NASCAR (for the most part, as far as I know) is a plug that's flush with the combustion chamber, with a fixed gap electrode. Instead of a strap they have a "disc" with a hole in it that the positive electrode comes through and is flush with the ground "disc" . . All the Gen II, III, and IV GM engines use them and when they switched over to Ethanol it didn't seem to necessitate a plug design change . . A friend of mine has a Gen II in his Bonneville car and I've tried to get the plugs he uses in it for our big block, but so far . . no luck . . And Champion (the manufacturer) doesn't even list them in their exotic racing plug catalog . . Knowing someone at Childress might be helpful, heh, heh . .